A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In device manufacturing methods using lithographic apparatus, an important factor in the yield, i.e. the percentage of correctly manufactured devices, is the accuracy within which layers are printed in relation to layers that have previously been formed. This is known as overlay and the overlay error budget will often be 10 nm or less. To achieve such accuracy, the substrate must be aligned to the mask pattern to be printed with great accuracy.
One known process for aligning the substrate and mask is known as off-line alignment and is performed in lithographic apparatus having separate measurement and exposure stations. It is a two-step process. First, at the measurement station, the positions of a plurality of, e.g. sixteen, alignment markers printed on the substrate relative to one or more fixed markers, known as fiducials and provided on the substrate table, are measured and stored. Then, the substrate table, with substrate still firmly fixed thereto, is transferred to the exposure station. The fiducial., as well as a marker detectable by an alignment sensor, also comprises a transmission image sensor (TIS). This is used to locate in space the position of an aerial image of a mask marker contained in the mask pattern that is to be exposed onto the substrate. Knowing the position of the TIS, and hence the fixed markers, relative to the image of the mask marker and also the positions of the substrate alignment markers relative to the fixed markers, it is possible to position the substrate in a desired position for correct exposure of the substrate to the mask pattern.
There are of course other known alignment methods, including through-the-lens methods in which an image of a grating mark on the mask is projected onto a grating mark on the substrate, or vice versa, using the projection system of the apparatus and alignment is detected by looking at the resulting diffracted light.
With the continual desire to image ever smaller patterns to create device with higher component densities, there is pressure to reduce overlay errors, which leads to a desire for improved alignment methods and systems.